Castable, synthetic resin materials currently on the market are perceived to be brittle in performance, particularly when they are compared with metals that have been traditionally used in dental and medical restorative applications. However, metal systems such as alloys and amalgams suffer from limited biocompatability and high heat and electrical conductivity, these deficiencies having negative health implications.
Dental composite resins have been developed from castable, synthetic resins especially for dental applications. Dental composite resins are characterized by extensive variation in the resin matrices utilized, ranging from acrylic through epoxy to recently developed urethane modified resins. Dental composite resin materials are easy to use and polish, they are photocurable and/or chemically curable and are relatively tough, strong and durable.
Dental composite resin materials are different from most castable, synthetic resin materials in that they are highly loaded (up to 90% and possibly further) with fine filler particles, and are predominantly photocured.
In general, dental composite resin materials include synthetic resins such as bis/GMA, urethane dimethacrylates, difunctional monomers such as TEDGMA; photoinitiators; amine accelerators; and polymerisation inhibitors and reinforcing fillers such as precipitated and pyrogenic silica, fine particles of glass (Ba, B, Sr, Al, Li, --SiO.sub.2) silica (quartz) or ceramic. Dental composite resins are classified as conventional, microfine or hybrid depending on the reinforcing fillers used.
A typical hybrid formula is 15% bis/GMA, 15% TEDGMA, 50% pyrogenic silica, 20% Ba glass and &lt;1% photoinitiators.
Biocompatibility is still being investigated, however, dental composite resins appear to be more biocompatible than metals.
The filler particles used are generally small (&lt;5 .mu.m) so as to enable greater filler loadings and enhance physical properties such as polymerisation shrinkage, plain strain fracture toughness (K.sub.1c) surface polishability and retained smoothness. Conventional dental composite resins with larger particles (&lt;30 .mu.m) cause unacceptable surface roughness of restorations (fillings) when the inevitable surface wear of the matrix resin occurs.
Existing dental composite resins do not have the necessary physical properties to be universally endorsed as amalgam replacements.
The current invention relates to the finding that the strength and toughness of castable, synthetic resinMs can be further improved by the incorporation of fibres or platelets or combinations thereof.
Particles can be defined as having similar extent in three dimensions whereas fibres have substantially greater extent in one dimension compared to two dimensions, and platelets have substantially greater extent in two dimensions compared to one dimension.
In particular, the strength and toughness of dental composite resins and their acceptability as amalgam replacements can be improved without substantially reducing the acceptability of the dental composite resin for curing and surface finishing, by the incorporation of fibres, platelets or combinations thereof. Fibres and platelets produce greater K.sub.1c than particles because of the higher energy required for debonding fibres or platelets compared to particles from a resin matrix. Hence more energy is required for fracture to occur.
Fibre reinforcement of castable, synthetic resins has been investigated [Krause et al "Mechanical properties of BIS-GMA resin short glass fiber composites", J. Biomed Materials Res. 23, 1195-1121 (1989)]. The authors of this paper were not able to demonstrate that improved strengths could be achieved in resins reinforced with fibres alone.